1. Technical Field
The invention relates to a method and system for indicating the position of control rods within a nuclear reactor. More particularly, the invention relates to a rod position indicator method and system which uses both analog and digital circuitry to determine the position of control rods within a nuclear reactor. Even more particularly the invention relates to a rod position indicator method and system which uses a ratiometric technique and simple linear correction circuitry to determine the position of a control rod within the core of a nuclear reactor.
2. Background of the Invention
Control rods, which generally include a cluster of elongated rods containing neutron absorbing materials, regulate the core activity within a nuclear reactor. The movable control rods are located within the region of the nuclear fissionable fuel and penetrate the core and fuel to a selected depth which is measured between 0 and 231 steps. When the control rods are inserted into the reactive region they absorb neutrons emitted from the fuel. The number of neutrons in the fuel determines the number of fissions of the fuel atoms that take place, and the number of fissions determines the amount of energy released by the nuclear reactor. Therefore, the number of control rods inserted and the depth of insertion can along with other measures control the amount of energy released by the nuclear reactor.
Energy in the form of heat is removed from the reactive region by a coolant which flows through the region and to a heat exchanger. The heat from the reactor coolant is used to generate steam for energy. Thus, reactor fuel consideration make it of the utmost importance to accurately know the position of each of the control rods within a nuclear reactor.
The nuclear fissionable fuel elements used in the reactor must be rearranged or replaced about every 1.5 years. After the refueling, all equipment must be tested and calibrated before the reactor can start producing energy. This period of testing and calibration is generally on the critical path to returning the plant to service after a refueling. The critical path includes, among other things, testing and calibrating the electronics which indicate the position of the control rods. As part of the indicator calibrations, the control rods must be fully inserted into the reactive core to calibrate a "zero" step position. The rods are then fully retracted from the reactor core and allowed to thermally stabilize before adjusting a "span" or 231 step position. The reactor may be powered up after these and all other tests and calibrations have been performed. Rod position is then continually monitored using the calibrated rod position indicator.
Rod position has been determined by a number of different types of indicators. One such indicator uses a plurality of layered coils concentrically wound in a stack and supported by a nonmagnetic stainless steel tubular substructure that is slid over a nonmagnetic rod travel housing. The coils are arranged alternately as primary and secondary coils and are connected in series independently from one another. The coils form a long linear voltage transformer distributed over the height of the travel housing such that the coupling from primary to secondary is affected by the extent to which the magnetic drive rod penetrates the coil stack. Rod position is determined by applying a constant sinusoidal excitation current to the primary coil stack and measuring the voltage induced across the secondary coil stack. The magnitude of the induced secondary voltage corresponds to the position of the rod and is processed by an indication system for display on a control panel.
Inherent drawbacks exist in the transformer type rod position indicator system, hereinafter referred to as a RPI system, and other prior art RPI systems. One of these drawbacks is that the reactor vessel contains a plurality of neighboring set of rod position sensing coils, each including primary and secondary coils. The stray magnetic flux by one primary coil induces a voltage on adjacent secondary coils. This induced voltage affects the output secondary voltage and provides an inaccurate reading of the control rod position.
Another drawback of the prior art RPI systems is that the secondary voltage drifts with changes in the operating conditions of the reactor. A principal source of this drift has been traced to changes in the permeability and resistivity of the drive rod with variations in drive rod temperature. This problem causes significant error in the system during reactor operation for rod movements and load changes.
A third drawback of the prior art RPI systems is the time required to calibrate the indicator. Past RPI systems couple the zero and span adjustments. The zero position is calibrated with the rod fully inserted into the reactor. When the rod is retracted to calibrate the span position, the rods have to be allowed to thermally stabilize for at least an hour before an accurate span position can be taken. Since adjusting the span changes the zero, it is necessary to repeat the calibration until the zero and span are within specifications. This process can take several days due to the one hour thermal stabilization period of the rods during the span adjustment and due to the time involved in continually moving the rods into and out of the reactor core.
Another drawback of the prior art RPI systems is the plurality of error sources to which the transformer circuitry is susceptible. These error sources include variations in primary excitation current caused by source variations, variations in the primary excitation current caused by changes in the magnetizing and leakage reactance of the coil stack which results from changes in rod position and core temperature, errors produced by harmonic content of the secondary signal produced by primary current distortion and non-linear aspects of the coil stack magnetics, errors caused by changes in primary circuit loop resistance due to temperature effects and connector contact resistance, and errors caused by changes in secondary loop resistance due to temperature effects and connector resistance variations.
Prior art RPI systems have attempted to solve some of these problems. U.S. Pat. No. 5,392,321 uses a plurality of differential amplifiers which determine both the voltage across the main secondary coil and the induced residual voltage across the adjacent secondary coil. The primary coil which corresponds to the adjacent secondary coil is turned off and a coupled voltage is generated equally on both the main secondary coil and the adjacent secondary coil. The voltage induced on the adjacent secondary coil is measured and subtracted from the voltage across the main secondary coil to produce a compensated voltage across the main secondary coil.
U.S. Pat. No. 4,714,926 uses primary and secondary coils along with a tertiary coil to determine the control rod position. The tertiary coil compensates for voltage drift due to temperature changes and two analog-to-digital converters feed a PROM which stores previously calculated rod position curves.
U.S. Pat. No. 4,631,537 discloses a method for temperature compensating a RPI system. This prior art system uses a direct temperature measurement along with either a plurality of amplifiers, a plurality of analog to digital converters and PROM chips or a combination of both to determine the control rod position.
Still another drawback with the prior art RPI systems is the impact on plant resources when installing upgraded versions of the prior art RPI systems. The design of the prior art RPI upgrade systems requires major rework of the entire RPI system cabinets. This rework translates into additional installation time and cost to accommodate the upgrades.
Calibration problems have historically accounted for three days of the critical path to start-up after routine refueling outages. Two of these three days are tied up in electronics calibration made overly time-consuming by the design of the electronics. Periodically, individual rod position indications go out-of-spec during reactor operation requiring flux maps and supplemental calibration checks to be performed in order to maintain confidence in actual rod position and continue operation. This out-of-spec condition, or the threat of it for rod positions near their technical specification limits, repeatedly have had a negative impact on unit maneuverability and, in the short-term, unit generation. These negative attributes equate to longer plant outages and an overall higher operating cost. These problems have plagued the nuclear power plant industry and defied solution for twenty years. Although the prior art RPI systems were adequate for the purpose for which they were intended, the RPI system of the present invention improves on some of the above mentioned deficiencies.
The RPI system of the present invention decouples the zero and span adjustments. Each adjustment only has to be calibrated once and calibrating one adjustment will not cause the other adjustment to go out-of-spec. This decoupling requires that the thermal stabilization period only be performed once, potentially shortening the period of calibration by two days.
These past RPI systems have provided marginal accuracy when processing the secondary voltage and converting it into a control rod position. The ratiometric technique used in the RPI system of the present invention compensates for variable and unpredictable resistances which occur in the cables which transmits the signals to and from the primary coil and secondary coils, respectively or in the cable connectors which connect the cables to the coils.
The RPI system of the present invention provides back-fit compatibility with existing units and installation requires only minimal modification to the existing card frame chassis wiring. The RPI system of the present invention was designed as a plug-in replacement for the existing devices. When the RPI system is initially installed, an EPROM chip is pre-programmed with data based on past performance and testing to provide for linear correction of the indicator position. Further rod specific corrections may be implemented with the reactor at power or on subsequent reactor outages based on data obtained from each rod during initial calibration checks. The EPROM will not have to be programmed again unless the coil stack or rod drive internals are replaced.
Thus, the need exists for a RPI system which will increase the overall accuracy of the control rod position indication, which utilizes ratiometric techniques to more accurately determine the coupling factor of the coil stack, thereby reducing errors introduced by non-linearities in the rod displacement sensor, and enhance system stability through the use of improved analog and digital signal processing circuits. All of which offers an elegant, cost-effective solution to inaccuracies in current RPI systems that can reduce initial and subsequent calibration time by at least two days, will require no change to coil stacks upon replacement of the existing RPI systems, offers back-fit compatibility with the existing units, and that includes a simple module substitution which can be accomplished with little impact on plant resources. There is no other such method or system of which we are aware which accomplishes these results.